In the prior art of control systems for gas turbine engine compressors, it is known to modulate a compressor bleed valve to control the pressure rise across the compressor for the purpose of safely operating the compressor in a region below the compressor's surge line. The compressor's surge line is a stability limit represented as a line drawn through a number of points on a graph produced by coordinates that are computed using the pressure rise across the compressor and the corrected airflow through the compressor. It is desirable to operate gas turbine engines close to the compressor's surge line in order to achieve good fuel economy. However, operating the compressor in the surge region cannot be tolerated inasmuch as surge can result in sudden thrust loss and/or engine overtemperature and possible engine shutdown. Compressor surges may cause damage to the engine either in the form of cracking of vanes and blades in the compressor or by affecting the engine hot section parts due to surge-induced overtemperatures in the turbines.
Typically, engine controls monitor various engine parameters and include schedules of other engine parameters which are used to automatically control the engine. More specifically, these controls generally account in various ways for the surge characteristics of the particular engine (with an adequate safety factor) for which the control is designed.
Control systems to recover from a surge condition are known in the prior art. For example, U.S. Pat. Nos. 4,864,813; 5,165,844; 5,165,845; and 5,375,412, all assigned to the assignee of the present application, disclose techniques for recovering from or rapidly correcting surge conditions. However, none of these patents disclose control systems that prevent the compressor from reaching the surge condition.
Control systems that prevent operation in the surge region are known. For example, U.S. Pat. No. 4,991,389 to Schafer, assigned to the assignee of the present application, discloses the maintenance of the compressor operating line at an approximately constant safe distance below the surge line over the entire operating range of the engine. The control system in the '389 patent modulates the compressor bleed valve between full open and full closed positions as a function of the rate of change of compressor speed biased by flight conditions and corrected for engine power level.
Another control system for bleed modulation is described in U.S. Pat. No. 4,756,152 to Krukoski et al., assigned to the assignee of the present invention. The control system in the '152 patent modulates the compressor bleed valve during steady-state engine operation in accordance with a particular schedule based on such parameters as altitude, air speed and engine power level. In contrast, during transient engine operation, the bleed valve position is a function of the ratio of actual rate of speed change of the compressor to a maximum scheduled rate of speed change of the compressor, biased to account for engine speed.
One drawback of both of these prior art methods of compressor bleed valve control is that the conservative (or excessive) bleeding of the compressor air causes the engine to run hotter, faster and burn more fuel than is necessary for certain flight conditions. However, heretofore it has been more acceptable to burn more fuel than is needed as opposed to the alternative of risking a compressor surge or potential engine shutdown by closing the bleed valve without sufficient surge margin.
Further, the aforementioned and other types of typical control systems generally use fixed schedules for engine operation with large surge margins built in. The large surge margins provide a safety margin that accounts for engine usage over time and the associated engine degradation. During normal engine operation with the compressor operating below the surge line, the compressor bleed valve remains closed. Typically, the compressor bleed valve is opened when the compressor operating line would cross over into the surge region during low thrust operation. Thus, to minimize the tendency of the compressor to surge, the bleed valve is opened and compressor bleed air is ducted overboard. A new engine with its relatively large surge margin can essentially be operated with the compressor bleed valve closed. However, as the engine is used over time, the margin between the operating line of the compressor and the surge line progressively decreases. Therefore, by initially providing a large surge margin in the preset schedule for a new engine, the control systems of the prior art account for engine usage and associated performance deterioration over time. However, a new engine with such a control system is thus burdened with an efficiency penalty associated with opening the bleed valve at a higher power than required just to take into account engine operation at a future time associated with a degraded or used engine.